Design and optimization of a bowtie dipole adjacent to dielectric material for through the wall imaging using hybrid electromagnetic computational analysis techniques
Knock and announce missions occur frequently on the urban battlefield. It would be advantageous to locate all of the humans inside an urban structure and obtain a "head count" prior to action. For this reason, we developed a through-wall radar sensor capable of locating moving targets through concrete-walled structures and of displaying the results (in range versus cross range) at a video frame rate of 10.8 Hz while the sensor is a safe distance from the wall. This sensor, approximately 2.25 m in length, would be mounted to a vehicle and driven near a building at a standoff range from which the user may detect the moving targets inside the building, as shown in Figure 1. The sensor uses a frequency-modulated, continuous-wave (FMCW) radar architecture operating at S-band, where some wall penetration is possible, with a center frequency of 3 GHz with a 2 GHz ultrawideband chirp. A narrowband, spatial frequency filter provides a range gate that eliminates the wall from the image, facilitating maximum receiver dynamic range to be applied to the target scene behind the wall. A time-division multiplexed (TDM), multiple-input, multiple-output (MIMO) array provides a lowest-cost, least complicated solution to a fully populated antenna aperture capable of near-field imaging. To achieve video-frame-rate imaging, a data pipeline and streamlined imaging algorithm were developed. Coherent frame-to-frame processing rejects stationary clutter, revealing the location of moving targets. In previous work, the switched-antenna-array, through-wall radar sensor was shown to be effective at imaging human targets through a 10 cm thick, solid concrete wall at a 6 m standoff range at the rate of one image The ability to locate moving targets inside a building with a sensor situated at a standoff range outside the building would greatly improve situational awareness on the urban battlefield. A radar imaging system was developed to image through walls, providing a down-range versus cross-range image of all moving targets at a video frame rate. This system uses an S-band, frequency-modulated, continuous-wave radar with a spatial frequency range gate coupled to a time-division multiplexed, multiple-input, multiple-output antenna array to rapidly acquire, process, and display radar imagery at a frame rate of 10.8 Hz. Maximum expected range through a 20 cm thick, solid concrete wall is 20 m. Measurements show that this system can locate humans (moving or standing still) behind 10 and 20 cm thick, solid concrete walls and through " cinder-block " walls.